This application relates to and claims priority to corresponding Germany Patent Application No. 102 00 366.1, which was filed on Jan. 8, 2002, and which is incorporated by reference herein.
1. Field of the Invention
The invention relates to a multichannel receiver system for angularly resolved laser ranging measurement, in which a laser light pulse from a laser using the pulse travel time method is used to measure the ranges of a plurality of target points by means of an array comprising a plurality of apertures, of a photodetector and of an evaluation circuit.
2. Description of the Related Art
In the case of pulse travel time methods, the time between the emission and the reception of a laser pulse reflected by the target is measured, the duration of whose pulse is typically a few nanoseconds.
Rangefinders that can determine the range of a plurality of target points with the aid of a single laser pulse are particularly well suited for a number of applications. It is possible by the accurate measurement of the range of a plurality of target points to derive statements on the shape and the type of the target that can render automatic detection of the target possible.
A multichannel receiver system for angularly resolved laser ranging measurement, in which a laser light pulse from a laser using the pulse travel time method is used to measure the ranges of a plurality of target points by means of an array comprising a plurality of apertures, of a photodetector and of an evaluation circuit, is disclosed, for example, in U.S. Pat. No. 4,380,391. Here, an array of optical fibers can be fitted in the receiving plane, the individual optical fibers thereof having different lengths and being coupled to a single photodetector. It is possible thereby to use a suitable time division multiplex method to assign the individual detector signals to the respective receiver visual fields. Only a single evaluation circuit is required in the case of this method. This eliminates the different tolerances on a plurality of evaluation circuits. A disadvantage to date has been the difficult assignment of the detector signals to the respective receiver visual fields in the case of marked differences in range between the respective target points. It can happen in general that the sequence of the detector signals no longer corresponds to the sequence of the fiber length, and that an accurate assignment of the pulses to the specific visual fields thereby becomes impossible. Existing solutions to this problem are, for example, the lengthening of the respective optical fibers (for example a few hundred meters), in order to obtain a better assignment thereby. The complexity thereby arising and the large overall size of the multichannel receiver systems are very disadvantageous.
Reference may be made to xe2x80x9cKrug et al., Application of a 6xc3x978 Silicon APD Array and Hybrid Electronics for Scannerless 3D Imaging Ladar, NATO/IRIS Active Systems, 1995, Vol. II, pages 79 to 89xe2x80x9d concerning the further general prior art with reference to the implementation of the multichannel receiver. Here, a photodiode array can be fitted directly in the receiving plane. Each individual photodiode of this array then has a dedicated evaluation circuit.
Moreover xe2x80x9cBurns and Yun, Compact, multichannel receiver using InGaAs APDs for single pulse, eye-safe, laser radar imagery, SPIE Vol. 3065, 1997, pages 22 to 29xe2x80x9d exhibits an array of optical fibers that is fitted in the receiving plane and whose individual optical conductors are coupled to discrete photodetectors. Each of these photodetectors has a dedicated evaluation circuit here, as well.
A disadvantage in the two previously mentioned systems relating to the further general prior art is that the respective evaluation circuits have different electronic components, and therefore different tolerances can occur in each case in the determination of the measuring points.
It is therefore the object of the present invention to provide a multichannel receiver system for angularly resolved laser ranging measurement that removes the abovedescribed disadvantages of the prior art, in particular a multichannel receiver system of small overall size and low complexity, in the case of which the signals detected with the aid of a photodetector can be uniquely assigned to the individual receiver visual fields, and an improved accuracy of the ranging measurement is achieved in conjunction with an identical evaluation circuit for all receiver visual fields.
This object is achieved according to the invention by virtue of the fact that the laser light pulses entering each of the apertures and reflected by the target points are distributed in a preselected distribution ratio between at least two optical fibers with an individual length which corresponds to a light travel time tn,m, are subsequently received by the photodetector and are processed in the evaluation circuit, a unique assignment of the pulses to the respective apertures being rendered possible by their spacings, specified in accordance with the distribution, and amplitudes, and it thereby being possible to determine the ranges of the target points.
A multichannel receiver system of small overall size and low complexity can be constructed by these measures, since only one photodetector and one evaluation circuit are required. Particularly advantageous is the possibility of uniquely assigning the signals detected with the aid of a single photodetector to the individual receiver visual fields. This is achieved in a simple way by virtue of the fact that for the purpose of assignment an individual item of additional information is impressed on the pulses owing to the specific length differences between the respective fibers and to the characteristic pulse amplitudes thereof, which are prescribed by the respective distribution ratios. A further advantage of the method stems from the fact that the statistical accuracy of the pulse travel time measurement, and thus of the ranging measurement, is increased in the case of measurement of a plurality of pulses from the same target.
Advantageous refinements and developments of the invention emerge from the subclaims and from the following exemplary embodiment described in principle below with the aid of the drawing.